The image intensifier has evolved rapidly from a simple diode to cascaded diodes, then to three element tubes with microchannel plate electron multipliers, and now to microchannel plate tubes with extremely sensitive III-V compound photocathodes. However, the vacuum hardware used for processing and assembly of the components into finished tubes has processed much more slowly. The present lack of sophistication in vacuum hardware is an impediment to both tube research and high volume production. A typical processing system is still configured to accept one set of components. These components are loaded into the system at atmospheric pressure. To begin processing, the system must be evacuated and undergo a high temperature bakeout to achieve an adequate vacuum. Thus, with this first design limitation, the fabrication cycle is limited to the three parts initially loaded: there is no opportunity to replace a component if any part should fail. Secondly, single assembly requires a complete pump down and bakeout, normally a 12 to 18 hour procedure. And finally, all parts must be cycled through a thermal bakeout whose primary function is degassing of the vacuum chamber.
A second present deficiency is the near total lack of any test capability for component monitoring during the fabrication process. Each part undergoes at least three different process steps before the tube is finished. There is only limited means to determine the success or failure of any step during processing. Conventionally, components are tested only before loading and after completion of the tube. Because of the many processing steps and the multiple components, determination of process control by evaluation of only the finished tube is difficult, if not impossible. Only after the fabrication of many tubes can conclusions be unambiguously drawn.
The third deficiency is inadequate separation of process steps. Many compromises must be made in the conventional chamber because of the proximity of the parts during the processing of any one of them. One example is that all parts are thermally degassed at the same temperature, namely that used to degas the chamber. A second example is that the formation of the photocathode causes vapor pressures which affect the performance of the other parts. Many other examples of processing compromises are known to those familiar with the art.
Besides impeding image intensifier development, the conventional chamber design constitutes a high cost for both development and production. Already discussed are the long cycling time for a single fabrication, the inability to substitute parts when there are failures, and the difficulty in optimizing process control. To offset these deficiencies, many duplicate vacuum chambers are used. At best, one chamber can handle one test intensifier or one production assembly per day. So to progress at a reasonable rate, an extremely large number of individual vacuum stations is required.